1. Field of the Invention
The present invention relates to a chemically processed steel sheet having a converted layer, which is excellent in workability and corrosion resistance at both a flat plane and a worked or machined part, generated on a surface of an Al—Si alloy plating layer.
2. Description of Related Art
Al-coated steel sheets have been used as steel material excellent in corrosion resistance. But, when the Al-coated steel sheet is held as such in a humid atmosphere, exhaust gas or an environment subjected to dispersion of sea salt grains for a long time, its external appearance is worsened due to generation of white rust on the Al plating layer. Chromating effectively inhibits generation of white rust on a surface of the Al-coated steel sheet for the following reasons.
A chromate layer generated on a surface of a steel base is composed of complex oxides and hydroxides of trivalent and hexavalent Cr. Scarcely-soluble compounds of Cr(III), such as Cr2O3, act as a barrier against a corrosive atmosphere and protect a steel base from corroding reaction. Compounds of Cr(VI) are dissolved as oxoatic anions such as Cr2O72− from the converted layer and re-precipitated as scarcely-soluble compounds of Cr(III) due to reducing reaction with exposed parts of a steel base formed by working or machining. Re-precipitation of Cr(III) compounds autogenously repairs defective parts of the converted layer, so that a corrosion-preventing effect of the converted layer is still maintained after working or machining.
Although chromating is effective for corrosion prevention of a steel sheet, it obliges a big load on post-treatment of Cr ion-containing waste fluid. In this regard, chemical liquors containing compounds such as titanium compounds, zirconium compounds or phosphates have been developed for generation of converted layers (hereinafter referred to as “Cr-free layers”), which do not contain chromium compounds or Cr ion, and some are already applied to aluminum DI (drawn and ironed) cans. For instance, JP 9-20984 A1 proposed an aqueous solution containing titanium compound, sulfuric phosphate, fluorides and an accelerator for coating an Al-containing metal part with a chemically converted (titanium compound) layer.
Titanium compound, zirconium compound or phosphate-containing converted layers, which have been proposed instead of the conventional chromate layer, do not exhibit such a self-repairing faculty as the chromate layer. For instance, a titanium compound layer does not exhibit a self-repairing faculty due to insolubility, although it is uniformly generated on a surface of a steel base in the same way as the chromate layer. As a result, the titanium compound layer is ineffective for suppression of corrosion starting at defective parts formed during chemical conversion or plastic deformation of a steel sheet. The other Cr-free layers are also insufficient for corrosion prevention due to poor self-repairing faculty.
When a small amount of a Cr-free chemical liquor is spread on an Al-coated steel sheet by a conventional method using an applicator roll or a spray wringer, an Al plating layer is not uniformly coated with a converted layer. The uncoated parts, i.e., surface parts where the Al plating layer is exposed to an atmosphere, act as starting points for corrosion or scratching during working, resulting in occurrence of damages in the converted layer or the Al plating layer. When a relatively thick converted layer is generated so as to completely cover the plating layer by spreading an excessive amount of a Cr-free chemical liquor, it does not work. On the contrary, defects such as cracks easily occur in the converted layer during press-working, since the converted layer cannot follow to deformation of a steel base. The defects, in addition to an insufficient self-repairing faculty, cause degradation of corrosion resistance.